Recently there is growing interest to develop wireless power transfer systems to power various devices from consumer electronics such as cell phones to heavy duty industrial equipment such as motors at the end of a crane. The main driving force behind this interest is the ability of such wireless power transfer systems to remove the direct electrical contact necessary to deliver power from a source to a load in a traditional wire-connected system. The removal of such electrical contact promise many advantages, including convenience, reduced maintenance cost, and reliability.
In a typical wireless power transfer system, shown in FIG. 1, a transmit inductor L1 is energized by a transmitter circuit 12, powered by a voltage source 13, to transmit a time varying magnetic field. A receive inductor L2 is then placed in the generated magnetic field, inducing a similarly time varying current that flows in the receive inductor L2. This induced current can be used to generate an AC voltage in a receiver circuit 14, which in turn can then be rectified at the receiver output to produce a DC voltage capable of delivering DC current to power a load RL on the receiver side. The load may be a battery to be recharged or a circuit. The receiver circuit 14 may be located inside the housing that also houses the load, such as a cell phone or other hand-held device. The transmitter circuit 12 may be located in a support platform on which the cell phone is placed, such as overnight, for recharging its battery.
In most practical wireless power transfer system, the amount of magnetic field that reaches the receiver inductor L2 is relatively small compared to traditional transformer-based isolated systems. An often used measure of how much magnetic field generated by the transmitter inductor L1 reaches the receiver inductor L2 is called coupling, represented by a coupling coefficient k between 0 and 1. Systems that have a coupling coefficient less than 0.8 often employ resonant circuits in the transmitter circuitry to generate enough current in the transmit inductor L1. This relatively large transmit inductor L1 current is required to generate the strong magnetic field needed to induce a sufficient current in the receive inductor to power the load.
Note that a resonance circuit is often also employed on the receive side. By tuning the resonance circuit on the receive side to the same frequency as the frequency at which the magnetic field is changing, the resonance circuit provides a preferred path for the magnetic field to close its loop (note that magnetic field lines always have to close a loop on itself since we do not have a magnetic monopole). Therefore, the resonance circuit at the receiver helps to reshape the local magnetic field around the receive inductor L2 and increases the field density such that a relatively larger amount of current can be induced in the inductor.
In a resonant receiver, this AC-current flows back and forth between the receive inductor L2 and a capacitor in the receiver circuit 14, generating a voltage. A larger induced current generates a larger peak voltage which can then be more easily rectified and potentially regulated, producing the desired voltage for the receiver load.
On the transmit side, a common resonance circuit used to generate the AC current in the transmit inductor is shown in FIG. 2. FIG. 2 illustrates a half bridge driver driving an LC tank circuit. Switches SW-A and SW-B are driven at a particular frequency and at a particular duty cycle. This frequency is usually determined by sweeping a certain frequency range and monitoring the voltage VL generated across the transmit inductor L (transmit inductor L1 in FIG. 1). When the amplitude of the voltage VL is at a maximum, the driving frequency is then determined to be equal to the natural frequency of the LC tank. The duty cycle of switch SW-A with respect to switch SW-B can also then be controlled to regulate the peak amplitude of VL, which corresponds to the peak amplitude of current IL at this LC tank natural frequency. Note that in normal operation, switches SW-A and SW-B are never on simultaneously. However, the presence of the diodes D1 and D2 allows the two switches to be off simultaneously while maintaining the SW node voltage within a diode drop of the supply voltage VS or ground.
Note that the LC tank circuit's natural frequency often does not stay fixed at one value during the course of the circuit operation. For example, the capacitor and transmit inductor used usually have a temperature coefficient, which means that the natural frequency of the tank will shift with temperature variation. And since we are developing a relatively large amount of AC current in this inductor and capacitor, a temperature increase of 20 to 30 degrees in this capacitor and inductor is relatively common. Another common effect is the shift in the natural frequency when a receiver inductor is coupled at different or changing coupling coefficients.
All these varying effects require that the frequency sweeping to find the natural frequency needs to be repeated at regular intervals to ensure that switches SW-A and SW-B are driven at the LC tank's natural frequency. The need to continually search for this natural frequency and the duty cycle modulation control usually means that complex digital circuitry is involved. More often than not this necessitates the use of a microprocessor to implement a custom algorithm.
What is needed is a transmission technique, for wirelessly transmitting power to a load, which sets the natural frequency of a tank circuit in a more cost-effective manner.
Another problem with prior art transmission systems for wirelessly transmitting power to a load is that the transmit power is typically set for a worse case load scenario. Therefore, if the load on the receiver side does not need much power, the transmit power is excessive and wasted. Accordingly, what is also needed is a technique that efficiently controls the amount of transmission power to only be that actually required for the load.